Gas hydrates are formed whenever water and hydrocarbon gases are combined under high pressure and low temperature.
Gas hydrates are crystal lattices made up of two or more constituents. The molecules of one component (always water) form a structure having relatively large cavities, which are occupied by the molecules of other constituents, these being separate gases or gaseous mixtures.
Gases which are important from the industrial as well the laboratory aspect show structures defined by the formula X.nH.sub.2 O where X is the hydrate-forming molecule while the number of water molecules in the compound is n&gt;5.67. Generally, hydrates are formed only in the presence of condensed water, that is, liquid water or ice. The water molecules linked by hydrogen bridges form a host (receiving) network around one or more species of the guest molecules. A physical encapsulation process occurs which is accompanied by weak interactions between the host-guest constituents when the guests enter the cavities of the host structure and are released therefrom under appropriate circumstances, by the collapse of the host structure.
Thus, the gaseous components within the cavities are not directly linked to the water molecules of the network. Due to geometrical reasons, such components cannot abandon the network of water molecules linked by hydrogen bridges until such network collapses.
Therefore, in the stable state, gas hydrates are always clathrate compounds of two or more components, since the components are mutually inserted via a complex mechanism. However the cohesion forces between the host and guest molecules do not suffice for forming a clathrate. Besides the cohesive forces, two basic criteria must be met in order to form a clathrate: the trend of water molecules to form a network must be satisfied, while the guest molecules must show suitable size and shape to enter the cavities of the hydrogen-bridged water network. A further requirement for forming the structure is that there should not be any chemical reaction between the guest molecules and the water molecules, that is, during crystallization, hydrolysis as well as hydration should be avoided in order to prevent a structure whose total energy would be lower than that of the clathrate.
Generally, it is considered that for the gas hydrates to occur the components or constituents should meet the following requirements: low solubility in water, sufficient volatility, homopolar character, not too large van der Waals forces, evaporation heat lower than 31,400 J mol.sup.-1 as well as boiling point lower than 60.degree. C., the hydrate-forming component being devoid of hydrogen atoms able to yield additional hydrogen bridges. Finally, the hydrate-forming gas should not be fairly soluble in either water, as are for example NH.sub.3 or HCl, or a water miscible liquid, for example CH.sub.3 OH.
Studies carried out in the field of gas hydrates indicate that the initial conditions for forming gas hydates are determined by the nature of the gas, the water state, the pressure and the temperature. The formation conditions are set in heterogeneous phase diagrams plotted as pressure vs. temperature.
The probability that a gas hydrate will be formed is as high as its stability. The stability degree of a gas hydrate and consequently its dissociation temperature are influenced by the molecular size and the geometric shape of the hydrate-forming components. Among the hydrocarbon hydrates, the more stable are those of propane and isobutane. The conditions for hydrate formation for a single- or multicomponent gaseous system are thus more or less altered by the presence of a third component. Generally it can be said that this effect depends on the gas composition, the density of the corresponding gas, the nature and amount of the substance which is altering the structural conditions in water, and on the pressure existing in the system. In the presence of electrolytes or polar solutes, the primary factors which act to alter the conditions of hydrate formation and dissociation are the structural variations which depend mainly on the solute pressure, temperature and composition and also on the energy variations of the interactions among molecules.
Researches has shown that any amount of electrolytes dissolved in water will lower the temperature of hydrate formation at a given pressure. In low amounts, alcohols increase the temperature of hydrate formation; however, for increasing amounts, such temperature is lowered. In this latter case it is hypothesized that structural cavities in water are partially occupied (for example, by methyl groups in the case of methanol) and thus an ordering of the hydrocarbon chains similar to that of ice is enhanced in the vicinity of the organic molecules. For higher amounts of alcohol, the clathrate-forming aggregates are broken, whereby the possibility of hydrate formation is decreased in the same way as in the case where the water structure is adversely affected by the presence of electrolytes. The inhibiting effect of electrolytes and alcohols is very important in the processes of production and transportation of natural gas, and may be extended to other processes as well.
Gas hydrates frequently occur during working out of subsea wells, mainly in deep-water wells. The gas hydrate deposits are mainly made up of petroleum gas and formation water or aqueous fluids generated by combined effects of turbulence, pressurization and cooling.
When the gas hydrate deposits are found in the production string or even in the surgency line, such deposits invariably cause the complete plugging of the production flow.
Under conditions of secondary recovery such as the method known as Water Alternating Gas (WAG) where water and gas are alternatively pumped into a reservoir through an injection well under conditions of low temperature and high pressure, the water-as mixture may form hydrates which can plug the injection well, bringing huge drawbacks to the well infectivity. It is then interesting to prevent the formation of these hydrates by heating the reservoir with the aid of the SGN of the present invention.
Also, under conditions of petroleum oil production, there are situations where the gas produced in the presence of cold water creates conditions of gas hydrate formation, which may plug the wet gas streamflow.
Still, the transportation of petroleum fluids along pipelines or lines from offshore equipments to shore facilities may generate conditions for the formation of gas hydrates, the flow of fluid throughout the pipeline or line being thus impaired.
In the natural gas industry the occurrence of gas hydrates is met on a day-by-day basis, since the thermo-hydraulic conditions for such are highly favored.
Therefore, various thermodynamic conditions are found which favor the occurrence of gas hydrates, in production as well as in the secondary recovery of oil as well as in the transportation of petroleum fluids, besides situations which can be found in the production of natural gas from petroleum reservoirs.
The usual practice to prevent gas hydrate formation is the addition to the aqueous fluid of an anti-freezing agent in amounts of 10 to 40% vol. Normally such agents are hydroxylated compounds such as primary alcohols in C.sub.1 -C.sub.4, besides glycols. In Brazil ethyl alcohol is usually employed, with good results and relatively low cost.
U.S. Pat. No. 5,460,728 teaches a process for the inhibition of the formation of gas hydrate in streams which contain low boiling hydrocarbons and water, these streams being displaced throughout a conduit or pipeline. The process comprises adding to the stream a nitrogen component in a sufficient amount to inhibit the formation of gas hydrates in the mixture at the temperature and pressure found in the conduit.
U.S. Pat. No. 5,232,292 teaches a process for the control of clathrate hydrates in fluid systems, the hydrates hindering the flow of fluid in a fluid system. The process comprises the contact of an additive with the clathrate mass. Preferably, the additive contains a cyclic chemical group having five, six and/or seven members. The additives include a poly(N-vinyl lactam) having molecular weight higher than 40,000, the polymer comprising a backbone, a first cyclic chemical grouping which extends from the backbone, and a second cyclic grouping extending from the backbone, the first cyclic grouping comprising a nonaromatic five-member organic heterocyclic ring having an internal amide, the second cyclic chemical grouping comprising a nonaromatic seven member organic heterocyclic ring having an internal amide, the polymer comprising a non-cyclic chemical group extending from the backbone. Representative polymers are N-vinyl pyrrolidone and hydroxyethyl cellulose, used alone or in combination.
U.S. Pat. No. 5,244,878 teaches a process for delaying and/or reducing the agglomeration tendency of hydrates in conditions under which a hydrate may be formed, which comprises adding to the hydrate-forming stream of gas and water an amphiphilic non-ionic compound chosen among the group of polyol esters and substituted or non-substituted carboxylic acids. The amphiphilic compound may be also an anionic amphiphilic compound.
U.S. Pat. No. 5,076,364 teaches a process for preventing gas hydrate formation in a gas well by injecting a carrier and an alcohol such as glycerol or a glycerol derivative into the well and connected facilities/pipelines.
U.S. Pat. No. 4,856,593 teaches, in a process for flowing through a pipeline a wet gas stream from an offshore producing well to shore under conditions of temperature and pressure conducive to the formation of gas hydrates, an improvement which comprises introducing in the wet gas stream a surface active agent of the group of organic phosphonates, phosphate esters, phosphonic acids, salts and esters of phosphonic acid, inorganic polyphosphates, esters of inorganic polyphosphates, polyacrylamides and polyacrylates in a sufficient amount to prevent stoppage of the flowing stream.
However, the control of gas hydrate formation by means of additives may be costly and of reduced efficacy.
On the other hand, the use of nitrogen gas and heat for various applications is well-known.
U.S. Pat. No. 4,846,277, of the Applicant and hereby fully incorporated as reference, teaches a continuous process for the hydraulic fracturing of a well with in situ nitrogen foam generation from the exothermic reaction between nitrogen inorganic salts, chiefly ammonium chloride and sodium nitrite, in the presence of a buffer which is able to keep the pH solution at 5.0 or less, and a viscosifying compound which may be any hydrosoluble polymer or gel which is able to increase the effective viscosity of the generated foam. The buffer system may be acetic acid at concentrations of from 0.5 volume % and the viscosifying compound is preferably hydroxyethyl cellulose (HEC). The polymeric viscosifier shows various advantages relative to the usual surface agents, since those may alter the rock wettability, emulsify when contacted with oil or precipitate if incompatible with the formation water. Further, the amount of polymeric viscosifier is less than that of surface agent for the same viscosifying degree.
U.S. Pat. No. 5,183,581 of the Applicant and hereby fully incorporated as reference, teaches a process for the dewaxing of producing formations based on a Nitrogen Generating System/Emulsion designed for the dewaxing of producing formations with the aid of nitrogen gas and heat generated by the reaction between aqueous solutions of nitrogen inorganic salts in the presence of an emulsified organic solvent. Paraffin deposits are typically made up of preferably linear, saturated hydrocarbon chains in C.sub.16 to C.sub.80 in admixture with branched hydrocarbons, asphaltenes, water and various mineral substances. The deposition phenomenon or precipitation of solid wax is an example of fluid/solid phase equilibrim, which can be explained in the light of principles of solution thermodynamics, that is, the solution of a hydrocarbon of higher molecular weight in hydrocarbons of lower molecular weight which function as solvents. That is, high molecular weight solids precipitate whenever the transport ability of the compound which works as solvent for the fluid is reduced.
U.S. Pat. No. 5,580,391 of the Applicant and hereby fully incorporated as reference teaches a process for the thermo-chemical cleaning of storage tanks which contain sludges from petroleum oil or related products. The process is carried out by the combined action of an organic solvent and the generation of nitrogen gas and heat, whereby is produced heating in situ, agitation by turbulence and flotation of the fluidized sludge, which after being collected and transferred to tanks or desalting units can be reintroduced in the usual refining flow.
U.S. Pat. No. 5,539,313 of the Applicant and hereby fully incorporated as reference teaches a process for the thermo-chemical dewaxing of hydrocarbon transmission conduits, which comprises treating the wax-containing conduit with a water-in-oil emulsion, co-currently to the production flow. The emulsion contains inorganic reactants which generate nitrogen and heat, which fluidize the paraffin deposit which is later driven off by cleaning beds.
The literature thus indicates on the one hand efforts for fluidize the gas hydrates by incorporating an additive to the oil or gas stream so as to alter the thermo-dynamic conditions of hydrate formation. On the other hand, the Applicant has developed a nitrogen and heat-generating treating fluid--the SGN fluid--which, by generating nitrogen and heat can possibly alter the thermo-hydraulic hydrate-forming conditions so as to prevent the formation or dissolve the hydrates which may form in producing wells, injection wells or reservoirs, as well as those formed in gas conduits submitted to conditions of hydrate formation.